workin on a 99 f150 sport new master cyl. new booster. no brake pedal good pedal when not runnin but nothing whe it is runnin. i think it is a vac problem but nobody wants to hear it what do you guys think??

thanks for the help (even if im not right lol)

 

Not a vacuum problem.

Sounds like air in the lines or maybe a bad master. Does the pedal slowly sink, or just drop to the floor? When you re-installed the booster, did you count the number of turns on the pushrod nut?